Pilot pressure operated directional control valve and an operating cylinder control apparatus

ABSTRACT

A pilot pressure operated directional control valve is provided in which a housing is formed therein with a spool having a plurality of ports. The spool bore has a spool fittingly inserted therein so as to be slidably displaceable therein and the spool is slidably displaced both by a spring and under a pilot pressure led into a pressure receiving chamber. Also a spring box is attached to an end surface of the housing in a longitudinal direction of the spool. The spring box is provided with a hollow portion which contains the spring while forming a pressure receiving chamber, as well as a pilot pressure fluid inlet that communicates with the hollow portion and is open in the longitudinal direction of the spool.

TECHNICAL FIELD

This invention relates to a pilot pressure operated directional controlvalve for switching the flows of a pressure fluid by slidably displacinga spool under a pilot pressure, and to an apparatus for controlling anoperating cylinder by using such a directional control valve in which aboom cylinder, an arm cylinder or a bucket cylinder of a power shovel orthe like operating cylinder is supplied with a pressure fluid so as tocontrol an operating extension and an operating retraction thereof.

BACKGROUND ART

A certain pilot pressure operated directional control valve has hithertobeen known as disclosed in Japanese Unexamined Patent Publication No.Hei 3-172602.

Thus, it is known that in such a directional control valve, as shown inFIG. 1 of the accompanying drawings hereof, a housing 1 is formedtherein with a spool bore 2 to which a pump port 7, a first and a secondactuator port 8 and 9, and a first and a second tank port 10 and 11 areopened. The spool bore 2 has a spool 3 fittingly inserted thereinwhereas the housing 1 has a pair of spring boxes 4 disposed at its lefthand side and right hand side, respectively. The spool 3 is adapted toassume a neutral position when it is energized by a spring 5 provided inone of the spring boxes 4 and is adapted to be slidably displaced eitheragainst, or in a cooperation with, a resilient force by the spring 5 totake a first position or a second position by supplying a pilot pressurefluid into one of pressure receiving chambers 6 formed in the left handside and right hand side spring boxes 4, respectively, to establish andblock communication between the pump port 7 and the first or secondactuator ports 8 or 9, and communication between the first or secondactuator ports 8 or 9 and the first or second tank ports 10 or 11.

This notwithstanding, however, such a pilot pressure operateddirectional control valve is constructed in such a way that the spring5, the one pressure receiving chamber 6 and a pilot pressure fluid inlet12 are formed successively on an axis that is coaxial with the spool 3.Since in addition the pilot pressure fluid inlet 12 must be made to havesuch a length that a piping joint such as an elbow can be in threadedengagement therewith, the corresponding spring box 4 needs to beproportionally lengthened. As a consequence, a problem arises in thatthe length of the entire directional control valve is necessarilyincreased and the area of the site on which it is mounted (i. e. a sitearea) is so enlarged.

Here, whilst if the pilot pressure fluid inlet is formed on a plane thatis orthogonal to the spool longitudinal direction of the spring box 4,the length of the spring box 4 can be shortened, then if a plurality ofsuch housings 1 are stacked one upon another and connected together, aproblem is brought about in that a piping joint as mentioned abovecannot be connected to the pilot pressure fluid inlet since theinterstice between the adjacent spring boxes 4 is necessarily small insize.

Furthermore, in such a pilot pressure operated directional controlvalve, it may be noted that in order for a pilot pressure fluid to besupplied into the pressure receiving chambers 6, a piping joint assemblysuch as including an elbow must be threaded into and thereby attached tothe pilot pressure fluid inlet 12 of the spring box 4.

For example, as shown in FIG. 2 of the accompanying drawings thereof,the pilot pressure fluid inlet 12 can be a threaded bore into which thethreaded portion 14 of a piping joint 13 may be inserted for a matingengagement therewith.

With such a construction adopted, in order for the piping joint 13 to befirmly attached to the said spring box 4 and for a fluid leakage fromtheir threaded engagement connection to be prevented, the threadedconnection length between the piping joint 13 and the pilot pressurefluid inlet 12 of the spring box 4 needs to be increased. Thus, thethreaded portion of the pilot pressure fluid inlet 12 of the spring box4 engaged with the piping joint 13 being lengthened, the length of thespring box 4 will be increased. For this reason, there has hithertodeveloped the problem that the entire size of a directional controlvalve of this type must be increased in length and the area of the siteon which it is mounted (i. e. the site area) needs to be enlarged.

Also, if the piping joint 13 has a pipe connecting portion 15 that isnot aligned linearly with the threaded portion 14 but its alignmenttherewith is L-shaped as shown, it is naturally difficult to orient thepipe connecting portion 15 in a desired direction. More specifically,whilst if a plurality of such directional control valves are stacked oneupon another, adjacent pilot pipe lines can readily be interconnected byorienting their corresponding pipe connecting portions 15 in anidentical direction, if a threaded attaching arrangement as mentionedabove is then employed for a piping joint 13, it is altogether possiblethat the pipe connecting portions 15 may not be oriented uniformly.

The present invention has been made to obviate such inconveniences andhas for its one object to provide a pilot pressure operated directionalcontrol valve which is capable of reducing the length of the entirepilot pressure operated directional control valve in the prior art,thereby reducing the site area; which, if a plurality of housings arestacked one upon another and thereby interconnected, is capable ofconnecting a piping joint to a pilot pressure fluid inlet; and which, ifa pipe connecting portion of a piping joint is L-shaped, is capable oforienting a plurality of such piping connection portions in an identicaldirection.

One may also note at this point that a conventional apparatus forcontrolledly supplying a pressure fluid into an operating cylinder istypically constructed in such a manner that the pressure fluiddischarged from a hydraulic pump is supplied into one of a first and asecond chamber of the operating cylinder under the control by a pilotpressure operated directional control valve while a pressure fluid isdischarged from the other of the first and second chambers into areservoir, thereby performing an extending or retracting operation ofthe operating cylinder.

In such an apparatus, the pilot pressure operated directional controlvalve has a valve block that is formed therein with a spool bore towhich a pump port, a first and a second actuator port and a tank portare opened. The spool bore has a spool fittingly inserted therein so asto be slidably displaceable therein. The spool can be switched from itsneutral position to its first or second position to establish acommunication between the pump port and one of the first and secondactuator ports and a communication between the tank port and the otherof the first and second actuator ports, thereby permitting the pressurefluid to be supplied and discharged as mentioned above. Also, the spoolcan be set at its neutral position to block each of these ports, therebypreventing a return fluid, caused to flow into the first or secondactuator port, from flowing out into the reservoir.

It should be noted, however, that between the spool and the spool borein the valve block there exists small clearance which may allow for afluid leakage and that a retention pressure would, owing to an externalload, be produced in the first or second chamber in the operatingcylinder. The retention pressure will then cause a portion of the returnoil to flow through the above mentioned clearance into the reservoir. Itfollows, therefore, that the operating cylinder will undesirably haveeither an extending operation or a retracting operation (hereinafterreferred to as a "spontaneous fall").

For this reason, the prior art has adopted an arrangement in which acircuit for interconnecting the retention pressure producing chamber ofthe operating cylinder and the actuator ports of the directional controlvalve is provided with a locking valve which will, when the directionalcontrol valve is at its neutral position, be closed to act to preventthe return fluid, out of the operating cylinder, from flowing into aactuator port of the directional control valve and thus to prevent aspontaneous fall. On the other hand, when the directional control valveis at its first or second position, the locking valve will be opened toallow the pressure fluid to flow between the directional control valveand the operating cylinder.

However, if such a locking valve is incorporated as mentioned above, theretention pressure will be abnormally elevated when the operatingcylinder is acted upon by an inertia load or an external force.

Thence, there has been known an operating cylinder controlling apparatusthat is designed to resolve this problem in the prior art, as disclosedin FIG. 3 of Japanese Unexamined Utility Model Publication No. Hei2-91201, which is provided with a locking valve, called a sequencevalve, in a circuit for interconnecting one of the actuator ports of thedirectional control valve and the retention pressure producing chamberof the operating cylinder, as well as a relieve valve in a circuitbetween the locking valve and the operating cylinder, in such a mannerthat if the retention pressure is elevated when the operating cylinderis acted upon by a load of inertia or an external force, the reliefvalve may provide a relieving action for the pressure fluid into thereservoir.

There has also be known another operating cylinder controllingapparatus, as disclosed in FIG. 1 of Japanese Unexamined Utility ModelPublication No. Hei 2-91201.

This latter known apparatus, as shown in FIG. 3 of the accompanyingdrawings hereof, is provided with a locking valve 206, then called alogic valve, in a circuit 205 for connecting one actuator port 202 of adirectional control valve 201 and a retention pressure producing chamber204 of an operating cylinder 203 together, as well as a pilot valve 209for establishing and blocking a communication between a spring chamber207 of the locking valve 206 and a reservoir 208. Then, the pilot valve209 has a pressure receiving chamber connected to a relief valve 210 andthe circuit 205 has a connection to a main relief valve 211. Here, ifthe retention pressure in the retention pressure producing chamber 204of the operating cylinder 203 is elevated, the pilot relief valve 210will act to relieve the pressure fluid to bring the pilot valve 209 toits communicating position with the pressure fluid relieved, therebycommunicating the spring chamber 207 of the locking valve 206 with thereservoir 208. The locking valve will thus be opened to communicate theretention pressure producing chamber 204 with the circuit 205. As aresult, the retention pressure within the operating cylinder 203 will berelieved through the main relief valve 211 and eventually set free.

Of the above mentioned two operating cylinder controlling apparatuses inthe prior art, if the former is adopted, it follows that the need tohandle a large volumetric flow of the pressure fluid requires a reliefvalve to be necessarily large in size and hence any combined unit of therelief valve and the locking valve to also be large in size.Furthermore, if a plurality of such operating cylinders are involved asis typically the case, a like plurality of such units need to beinstalled, thus giving rise to the problematical requirement that thearea for their installation be necessary which needs to be increased inproportion to the number of the units.

On the other hand, if the latter type of the control apparatus isadopted, it follows that the use of the locking valve 206, the pilotvalve 209 and the pilot relief valve 210, an increased number ofcomponents, makes the equipment not only costly but also large in size.Therefore, the problem of the requirement of an enlarged installationsite, here again, arises, if a plurality of operating cylinders need tobe controlled as in the previous case.

Accordingly, the present invention has been made also to obviate theinconveniences discussed in the preceding paragraphs and has for anotherobject to provide an operating cylinder controlling apparatus which isreduced both in its cost and area of installation for a unit designed toprevent any spontaneous fall of an operating cylinder.

SUMMARY OF THE INVENTION

In order to achieve the foregoing objects, there is provided inaccordance with the present invention, in a first aspect thereof, apilot pressure operated directional control valve in which a housing isformed therein with a spool bore having a plurality of ports. The spoolbore has a spool fittingly inserted therein so as to be slidablydisplaceable therein and the spool is adapted to be slidably displacedboth with a spring and under a pilot pressure led into a pressurereceiving chamber, and in which:

a spring box is attached to an end surface of the housing in alongitudinal direction of the spool; and the spring box is adapted to beformed therein with a hollow portion containing a spring and forming apressure receiving chamber, and a pilot pressure fluid inlet forcommunicating with the hollow portion and opening in the longitudinaldirection of the spool.

According to the construction mentioned above, it can be seen that thepilot pressure fluid inlet will no longer project largely from thehollow portion in the longitudinal direction of the spool and, as aresult, the length of the spring box may be reduced, This will in turnshorten the entire pilot pressure operated directional control valve,thus reducing its required site area.

Also, since the pilot pressure fluid inlet is opening in the directionof the spool, it can be seen that if a plurality of housings are stackedone upon another and thereby interconnected, an individual pilotpressure fluid inlet may have a corresponding piping joint connectedthereto.

Preferably, the spring box comprises a first cylindrical body and asecond cylindrical body which are integrally arranged in a pair and inparallel to each other and which are opening to a left hand side andright hand side which are opposite to each other. The hollow portion isformed within the first cylindrical body, and an interior of the secondcylindrical body is adapted to be in a communication with an interior ofthe first cylindrical body. Also, the pilot pressure fluid inlet isconstituted with an opening portion of the second cylindrical body.

It is also preferred that a piping joint should be connected to thepilot pressure fluid inlet.

It is further desirable that the pilot pressure fluid inlet be formedwith a piping joint attachment hole, that the piping joint attachmenthole be constituted of a large diameter hole and a small diameter holewhich are eccentric to each other, that a piping attachment joint beconstituted of a fitting portion comprising a large diameter portion anda small diameter portion which are eccentric to each other and a pipeconnecting portion, that the fitting portion be adapted to be fittinglyinserted into the piping joint attachment hole, and that a pressureplate be bolted with the spring box to act to prevent the fittingportion from coming out of the piping joint attachment hole.

According to the construction just mentioned above, it can be seen thatthe length along which the fitting portion of the piping joint isfittingly inserted into the piping joint attachment hole of the springbox may be shortened. Since the piping joint attachment hole of thespring box can thus be reduced in length, the length of the spring boxwill be shortened, thereby reducing the entire length of the pilotpressure operated directional control valve, thus making its requiredsite area smaller.

It can also be seen that with such a pressure plate the piping joint mayeffectively be prevented from coming out of the piping joint attachmenthole. Also, since a large diameter portion and a small diameter portionwhich are eccentric to each other are fitted, respectively, into a largediameter hole and a small diameter hole which are eccentric to eachother, there will be no rotation of the piping joint and its firmattachment will thereby be ensured. If an L-shaped piping joint isadopted, it will be noted that its pipe connecting portion can always beoriented in a predetermined direction.

It is also possible that the pilot pressure fluid inlet may be formedwith the piping joint attachment hole, that the piping joint attachmenthole may be in the form of a regular polygon, that the piping joint maybe constituted of a fitting portion in the form of a regular polygon anda pipe connecting portion, that the fitting portion may be adapted to befittingly inserted into the piping joint attachment hole, and that apressure plate may be bolted with the spring box to act to prevent thefitting portion from coming out of the piping joint attachment hole.

According to the construction just mentioned above, it can be seen thatthe orientation of the pipe connection portion of the piping joint maybe altered in accordance with a particular regular polygonalconfiguration and thus by the number of corners of the particularregular polygon.

It should be noted at this point that it is desirable that an intersticebetween an inner peripheral portion of the joint attachment hole and anouter peripheral portion of the fitting portion be adapted to be sealedby a sealing material, or that an interstice between a bottom portion ofthe piping joint attachment hole and an end surface of the fittingportion be adapted to be sealed by a sealing material.

The present invention also provides, in a second aspect thereof, anoperating cylinder control apparatus, which comprises:

a directional control valve;

a locking valve which is disposed in a circuit for interconnecting anactuator port of the directional control valve and a retention pressuregenerating chamber of the operating cylinder, which has a pressurereceiving portion and a spring, which is adapted to be thrust in adirection of communication by an outlet pressure of the directionalcontrol valve and the retention pressure within the operating cylinderand which is adapted to be thrusted in a blocking direction by theretention pressure within the operating cylinder and the spring. Theretention pressure acts on the pressure receiving portion.

A switching valve is disposed between the pressure receiving portion ofthe locking valve and a reservoir, which is adapted to be energized bythe spring to take its blocking position and which is adapted to takeits communicating position by means of a switching means brought into aposition for communicating the circuit with a reservoir.

Also a main relief valve is connected via a check valve to a side to theoperating cylinder of the rocking valve in the circuit.

According to the construction just mentioned above, it can be seen thatsince the main relief valve for preventing an abnormally elevatedpressure within the retention pressure generating chamber of theoperating cylinder can be placed separately, it will be sufficient toprovide only the locking valve and the switching valve correspondinglyto each of the operating cylinders. And yet, since the main relief valvecan commonly act for a plurality of operating cylinders, the area of thesite on which a unit for preventing a spontaneous fall of any of theplural operating cylinders is mounted may be reduced. Also, since itsuffices to provide such a single main relief valve alone, it can beseen that the equipment may be made less costly.

In the construction just mentioned above, it should be noted here thatit is desirable that a valve block equipped with the locking valve andthe switching valve be connected to a valve block of the directionalcontrol valve, that an inlet side of locking valve be adapted tocommunicate with the actuator port of the directional control valve, andthat each of the valve blocks be formed therein with a fluid bore forcommunicating the pressure receiving portion on which the pilot pressureof the directional control valve is acting with the pressure receivingportion of a side to the spring of the switching valve.

It should further be noted that an operating cylinder control apparatusas mentioned may specifically comprise:

a directional switch valve which is provided with a pump port, a tankport and a first and a second actuator port, which when at its neutralposition is adapted to block the first and second actuator ports, whichwhen at its first pressure fluid supply position is adapted tocommunicate between the pump port and the first actuator port and tocommunicate between the second actuator port and the tank port, andwhich when at its second pressure fluid supply position is adapted tocommunicate between the pump port and the second actuator port and tocommunicate the first actuator port and the tank port.

A first circuit is provided for connecting the first actuator port to aretention pressure generating chamber of the operating cylinder.

A second circuit is provided for connecting the second actuator port tothe other chamber of the operating cylinder.

A locking valve is disposed in the first circuit. The locking valve isadapted to be thrust in a direction of communication both under apressure of the first actuator port and under a pressure within theretention pressure generating chamber. Also, the locking valve isadapted to be thrust in a direction of blocking both by a spring andunder the pressure within the retention pressure generating chamberacting on a pressure receiving portion.

A switching valve is disposed in a drain path connected to the pressurereceiving portion of the locking valve. The switching valve is held at ablocking position by a spring, and is brought to a communicatingposition under a pressure at the pressure receiving portion.

Also a main relief valve is connected to a side to the retentionpressure generating chamber of the locking valve in the first circuitand to the second circuit via respective check valves, and may have aconstruction in which the second pressure receiving chamber of thedirectional control valve is connected to the pressure receiving portionof the switching valve.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will better be understood from the followingdetailed description and the drawings attached hereto showing certainillustrative embodiments of the present invention. In this connection,it should be noted that such embodiments as illustrated in theaccompanying drawings are intended in no way to limit the presentinvention, but to facilitate an explanation and understanding thereof.

In the accompanying drawings:

FIG. 1 is a cross sectional view of a pilot pressure operateddirectional control valve in the prior art;

FIG. 2 is a cross sectional view of a piping joint attachment portion inthe above mentioned example in the prior art;

FIG. 3 is a hydraulic circuit diagram of an operating cylinder controlapparatus in the prior art;

FIG. 4 is a cross sectional view of a first embodiment of a pilotpressure operated directional control valve according to the presentinvention;

FIG. 5 is a left hand side view of the structure shown in FIG. 4;

FIG. 6 is a cross sectional view taken along the line VI--VI of FIG. 5;

FIG. 7 is a cross sectional view of a second embodiment of a pilotpressure operated directional control valve according to the presentinvention:

FIG. 8 is a cross sectional view of a third embodiment of a pilotpressure operated directional control apparatus according to the presentinvention;

FIG. 9 is a left hand side view of the structure shown in FIG. 8;

FIG. 10 is a front view of a piping joint in the above mentioned thirdembodiment of the present invention;

FIG. 11 is a cross sectional view of a fourth embodiment of a pilotpressure operated directional control valve according to the presentinvention;

FIG. 12 is a front view of a piping joint in the above mentioned forthembodiment of the present invention;

FIG. 13 is a cross sectional view of a fifth embodiment of a pilotpressure operated directional control valve according to the presentinvention;

FIG. 14 is a cross sectional view taken along the line XIV--XIV of FIG.13:

FIG. 15 is a hydraulic circuit diagram of an operating cylinder controlapparatus which constitutes a sixth embodiment of the present invention;

FIG. 16 is a cross sectional view illustrating a specific structure ofthe directional control valve for use in the above mentioned sixthembodiment of the present invention; and

FIG. 17 is a right hand side view of the structure shown in FIG. 16.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, suitable embodiments of the present invention with respectto a pilot pressure operated directional control valve and an operatingcylinder control apparatus using the same will be set forth withreference to the accompanying drawings hereof.

As shown in FIG. 4, a housing 20 is formed therein with a spool bore 21to which a pump port 23, a first and a second load pressure detectingport 24 and 25, a first and a second actuator port 26 and 27 and a firstand a second tank port 28 and 29 are opened. The spool bore 21 has aspool 22 fittingly inserted therein. The above mentioned spool 22 isformed with a first and a second small diameter portion 30 and 31 and anintermediate small diameter portion 32 so that when located at itsneutral position as shown in FIG. 4, it may block each of the portsmentioned above. And, when the spool 22 is slidably displaced rightwardsin FIG. 4 to take its first position, communication will be establishedeach between the pump port 23 and the second load pressure detectingport 25, between the first load pressure detecting port 24 and the firstactuator port 26, and between the second actuator port 27 and the secondtank port 29. It should be noted here that the first load pressuredetecting port 24 and the second load pressure detecting port 25 remaincommunicated with each other at all times. It can accordingly be seenthat a pressure fluid caused to flow into the pump port 23 will flowthrough the first actuator port 26 into an actuator 33 whereas a returnfluid out of the actuator 33 will flow through the second actuator port27 into the tank port 29.

Also, if the spool 22 is slidably displaced from the state of FIG. 4leftwards to take its second position, communication will be establishedeach between the pump port 23 and the first load pressure detecting port24 and between the second load pressure detecting port 25 and the secondactuator port 27. Since the first load pressure detecting port 24 andthe second load pressure detecting port 25 remain in communication witheach other at all times as mentioned above, the pressure fluid in thepump port 23 will flow into the actuator 33. Also, since the firstactuator port 26 is caused to communicate with the first tank port 28,the return fluid out of the actuator 33 will flow into the first tankport 28 mentioned above.

At this point it should be noted that in FIG. 4, a spool 34 is insertedin a check valve bore 20c and constitutes a check valve component 35whereas a spool 36 is inserted in a pressure reduction valve bore 20dand constitutes a pressure reduction valve component 37. The check valvecomponent 35 and the pressure reduction valve component 37 togetherconstitute a pressure compensation valve.

It is seen that a first spring box 40 is attached to one end surface 20aof the above mentioned housing 20 in the longitudinal direction of thespool 22. As shown in FIGS. 5 and 6, the first spring box 40 comprises apair of a first cylindrical body 42 having an attachment seat 41 and asecond cylindrical body 43 which is integrally mounted with, and extendsin parallel to, the first cylindrical body 42 and which is opening toboth a left hand side and a right hand side which are opposite to eachother. The first cylindrical body 42 is secured by a bolt 44 to thehousing 20 coaxially with the spool bore 21 and is provided therein witha first spring bearing 45 and a second spring bearing 46 so that thefirst spring bearing 45 may be in contact with both the one end 20a ofthe housing 20 and a step portion 22a of the spool 22 whereas the secondspring bearing 46 may be in contact with both a bottom wall 42a of thefirst cylindrical body 42 and a step portion 48 of a bolt 47 secured to,or integral with, the spool 22. Here, a spring 49 is interposed betweenthe first and second spring bearings 45 and 46 so that the spool 22 maybe held at its neutral position. Note also that the interior of thefirst cylindrical body 42 constitutes a pressure receiving chamber 50.Thus, not only does the first cylindrical body 42 serve to contain thespring 49 but also it is provided therein with a hollow portion 40a thatconstitutes the pressure receiving chamber 50.

An outer end portion of the above mentioned second cylindrical body 43is configured to open in a direction that is parallel to thelongitudinal direction of the spool 22, and a portion at which the body43 is open constitutes a pilot pressure fluid inlet 51. Also, an innerend portion of the second cylindrical body 43 is configured tocommunicate via a small diameter bore 52 with the interior (i. e., thepressure receiving chamber 50) of the first cylindrical body 42. Notefurther that the pilot pressure fluid inlet 51 is formed with a threadedportion 53, and that the pilot pressure fluid inlet 51 is located at anapproximately identical position to the end of the pressure receivingchamber 50 in the direction in parallel to the longitudinal direction ofthe spool 22.

A first threaded portion 55 of a piping joint 54 such as an elbow isinserted into the threaded portion 53 of the pilot pressure fluid inlet51 of the second cylindrical body 43 to establish a mating engagementtherewith. A second threaded portion 56 of the piping joint 45 is fittedwith an interiorly threaded pipe for a pilot pressure fluid to establisha mating connection therewith.

The first threaded portion 55 and the second threaded portion 56mentioned above are configured to be L-shaped as a whole to enable thepipe for the pilot pressure fluid to be connected approximately inparallel to the one end surface 20a of the housing 20.

It is also seen that a second spring box 60 is attached to the other endsurface 20b of the housing 20 in the longitudinal direction of the spool22. The second spring box 60 is only formed with a pilot pressure fluidinlet 61 and a pressure receiving surface 62 and does not contain aspring.

That is to say, since the spool 22 can be held at its neutral positionby the spring 49 disposed in the first spring box 42 and is alsoarranged to be slidably displaceable both leftwards and rightwardsagainst the spring 49, there is no need to provide a spring within thesecond spring box 60. In this connection it should be noted that withinthe second spring box 62 there is provided a seat 63 that is designed toregulate a slidable displacement rightwards of the spool 22.

This being the case, it can be seen that the second spring box 60 ifprovided in the form of a cylindrical body may have a length that isreduced by the space in which a spring otherwise needs to be contained.

This notwithstanding, however, it is not definitely objectionable toreplace the second spring box 60 with the first spring box 40 as one isto be attached there.

FIG. 7 shows a second embodiment of the pilot pressure operateddirectional control valve according to the present invention. In thesecond embodiment, the one end surface 20a and the other end surface 20bof a housing 20 have each a first spring box 40 attached thereto and aspring 49 is contained in a first cylindrical body 42 for each suchfirst spring box 40.

As will be apparent from the foregoing, if either the first or thesecond embodiment mentioned above is adopted in which there is includeda pilot pressure fluid inlet 51 that is opening in a directionsubstantially in parallel to a hollow portion 40a which contains aspring 49 and constitutes a pressure receiving chamber 50 and in whichthe pilot pressure fluid inlet 51 is located at a substantiallyidentical position to the end of the pressure receiving chamber 50 in adirection in parallel to the longitudinal direction of a spool 22, itcan be seen that there will be no undesirable, large projection of thepilot pressure fluid inlet 51 from the hollow portion 40a in thelongitudinal direction of the spool and, as a result, the length of thespring box 40 may be shortened. This will in turn shorten the entirelength of a pilot pressure operated directional control valve, thusreducing its required site area.

Also, since the pilot pressure fluid inlet 51 is designed to open in adirection in parallel to the spool longitudinal direction, it can beseen that even in case a plurality of housings are stacked one uponanother and thereby interconnected, such an individual pilot pressurefluid inlet 51 may have a piping joint readily connected thereto.

FIG. 8 shows a third embodiment of the pilot pressure operateddirectional control valve according to the present invention.

As shown in FIG. 8, a housing 120 is formed therein with a spool bore121 to which a pump port 123, a first and a second load pressuredetecting port 124 and 125, a first and a second actuator port 126 and127 and a first and a second tank port 128 and 129 are opened. The spoolbore 121 has a spool 122 fittingly inserted therein. The spool 122 isformed with a first and a second small diameter portion 130 and 131 andan intermediate portion 132 so that when held at its neutral position itmay block each of the ports mentioned above. And, if the spool 122 isslidably displaced rightwards in FIG. 8 to take its first position,communication will be established each between the pump port 123 and thesecond load pressure detecting port 125, between the first load pressuredetecting port 124 and the first actuator port 126, and between thesecond actuator port 127 and the second tank port 129. It should benoted at this point that the first load pressure detecting port 124 andthe second load pressure detecting port 125 remain communicated witheach other at all the times. Accordingly, it can be seen that a pressurefluid caused to flow into the pump port 123 will flow through the firstactuator port 126 into an actuator 133 whereas a return fluid out of theactuator 133 will flow through the second actuator port 127 into thesecond tank port 129.

If the spool 122 is slidably displaced from the state of FIG. 8leftwards to take its second position, communication will be establishedeach between the pump port 123 and the first load pressure 124 andbetween the second load pressure detecting port 125 and the secondactuator port 127. Also, since the first load pressure detecting 124 andthe second load pressure detecting port 125 remain in communication witheach other at all times as mentioned above, it can be seen that thepressure fluid out of the pump port 123 will flow into the actuator 133.Then, also, the first actuator port 126 will communicate with the firsttank port 128 to allow the return fluid out of the actuator 133 to flowinto the first tank port 128.

It is seen that a first spring box 140 is attached to one end surface120a of the above mentioned housing 120 in the longitudinal direction ofthe spool 122. The first spring box 140 is formed with a piping jointattachment hole 141 that is opening to one end surface 140a thereof, aspring attachment hole 142 that opens to the other end surface 140bthereof and a bore 143 for communicating these attachment holes 141 and142 with each other. It is seen that a first spring bearing 144 and asecond spring bearing 145 are disposed within the spring attachment hole142 so that the first spring bearing 144 may be in contact with both theone end surface 120a of the housing 120 and a step portion 122a of thespool 122 whereas the second spring bearing 145 may be in contact withboth a bottom wall 142a of the spring attachment hole 142 and a stepportion 147 of a bolt 146 secured to or integral with the spool 122. Itis also seen that a spring 148 is interposed between the first andsecond spring bearings 144 and 145 so that the spool 122 may be held atits neutral position. Also, the interior of the spring attachment hole142 here constitutes a pressure receiving chamber 149.

As shown in FIGS. 8 and 9, the spring attachment hole 142 comprises alarge diameter hole 150 and a small diameter hole 151 which areeccentric to each other. The large diameter hole 150 is open to the oneend surface 140a of the spring box 140 whereas the small diameter hole151 has a bottom portion thereof which is opening through the bore 143to the spring attachment hole 142.

As shown in FIG. 10, a piping joint 152 comprises a fitting portion 153and a pipe connecting portion 154 which are configured to be L-shaped asa whole. The fitting portion 153 comprises a large diameter portion 155and a small diameter portion 156 which are eccentric to each other. Thelarge diameter portion 155 is formed with an annular groove 157 on itsouter peripheral surface.

As shown in FIGS. 8 and 9, the piping joint 152 has such a constructionthat the large diameter portion 155 and the small diameter portion 156may be fitted in the large diameter hole 150 and the small diameter hole151, respectively, of the first spring box 140, and thus is attached tothe latter so that it may not be rotated. Also, an O-ring 158, which isfitted in the annular groove 157 of the large diameter portion 155, ispressed against, and thereby attached to, the inner peripheral surfaceof the large diameter hole 150 to provide a sealing between the pipingjoint 152 and the first spring box 140. Also, a pressure plate 160,which is attached by bolts 159 to the one end surface 140a of the firstspring box 140, serves to prevent the piping joint 152 inserted fromcoming out.

This being the case, it will be seen that the attachment portion of thepiping joint 152 to the first spring box 140 is sealed by the O-ring158, the piping joint 152 inserted can be prevented from being removedby the pressure plate 160, and the piping joint 152 can be fixed inposition so as to be not rotatable by means of the large diameter hole150 and the small diameter hole 151 which are eccentric to each other,coupled with the large diameter portion 155 and the small diameterportion 156 which are eccentric to each other, and yet the orientationof the pipe connecting portion 154 of the piping joint 152 can bemaintained always constant.

It is also seen that a second spring box 161 is attached to the otherend surface 120b of the above mentioned housing 120 in the longitudinaldirection of the spool 122. The second spring box 161, as with the firstspring box 140, is provided with a piping joint attachment hole, heredesignated at 141, and a spring attachment hole, here designated at 142.The piping joint attachment hole 141 has a small diameter hole 151 thatis directly open to the spring attachment hole 142 in which no spring isprovided.

That is to say, since the spool 122 can be held at its neutral positionby the spring 148 disposed within the first spring box 140 and is alsoarranged to be slidably displaceable both leftwards and rightwardsagainst the spring 148, there is no need to provide a spring within thesecond spring box 161. In this connection, it should be noted that as ina previous embodiment, here again, a piping joint 152 is attached to thepiping joint attachment hole 141, in the second spring box 161.

This being the case, it can be seen that the second spring box 161 mayhere again have a length that is reduced by the space in which a springmust otherwise be contained. This notwithstanding, however, it is notobjectionable to replace the second spring box 161 with the first springbox 140 as one is to be attached there.

It should also be noted that the piping joint 152, as in a fourthembodiment of the pilot pressure operated directional control valve asshown in FIGS. 11 and 12, may have a fitting portion 153 and a pipeconnecting portion 154 linearly arranged.

As will be apparent from the preceding paragraphs, the above mentionedthird and fourth embodiments of the present invention provide anarrangement whereby it is made possible to shorten the length alongwhich the fitting portion 153 of the piping joint 152 is fitted into thepiping joint attachment hole 141 of the spring box 140 while providing arequired sealing with certainty. Therefore, since the piping jointattachment hole 141 of the spring box 140 can be shortened, it followsthat the length of the spring box 140 will be shortened to reduce theentire length of the pilot operated directional control valve, therebyreducing its site area as required.

Also, the piping joint 152 inserted can be prevented by the pressureplate 160 from coming out. Further, since the large diameter portion 155and the small diameter portion 156 which are eccentric to each otherare, respectively, fitted in the large diameter hole 150 and the smalldiameter hole 151 which are eccentric to each other, it will be seenthat the piping joint 152, without any fear of its subsequent rotation,can be, and does remain, attached firmly in place. In other words, theorientation of the pipe connecting portion 154 of the L-shaped pipingjoint 152 may be maintained always constant.

Also, as in a fifth embodiment of the pilot pressure operateddirectional control valve as shown in FIGS. 13 and 14, a piping jointattachment hole 141 of each of the first and second spring boxes 140 and161 may be in the form of a regular polygon, and the fitting portion 153of the piping joint 152 may also be in the form of a regular polygonthat is identical to the above mentioned regular polygon. In this case,an O-ring 158 is fitted between the bottom portion of the piping jointattachment hole 141 and the end surface of the fitting portion 153.Then, the orientation of pipe connecting portion 154 of the piping joint152 may be altered in accordance with the configuration of a particularregular polygon and thus by the number of corners of the particularpolygon.

In this case, it should be noted that although a bolt 159 becomesnecessary which is capable of fastening a pressure plate 160 with anintensive force in order to squeeze the O-ring 158 to a sufficientdegree, such a bolt will not give rise to any problem whatsoever sinceit will not project from the piping joint 152.

According the above mentioned fifth embodiment of the present inventionin this way, it becomes possible to shorten the length along which thefitting portion 153 of the piping joint 152 is fitted into the pipingjoint attachment hole 141 of the spring box 140 while providing arequired sealing with certainty. Therefore, since the piping jointattachment hole 141 of the spring box 140 can be shortened, it followsthat the length of the spring box 140 will be shortened to reduce theentire length of the pilot operated directional control valve, therebyreducing its site area as required.

Also, the piping joint 152 inserted can be prevented from being removedby the pressure plate 160. Further, since the fitting portion 153 in apolygonal configuration is fitted in the piping joint attachment hole141 in a polygonal configuration, it will be seen that the piping joint152, without any fear of its subsequent rotation, can be, and doesremain, attached firmly in place. In other words, the orientation of thepipe connecting portion 154 of the L-shaped piping joint 152 may bemaintained always constant. And yet, it is also possible to alter itsorientation as desired.

FIG. 15 is a hydraulic circuit diagram of an operating cylinder controlapparatus that represents a sixth embodiment of the present invention.As shown in FIG. 15, a hydraulic pump 220 has its discharge path 221that is provided with a plurality of pressure compensation valves 222,each of which has an output side provided with a directional controlvalve 223. The directional control valve 223 is designed to establishand block communications among a pump port 224, a tank port 225, a firstand a second actuator port 226 and 227 and a load pressure detectingport 228. The first actuator port 226 of the directional control valve223 is connected to a retention pressure generating chamber 231 of anoperating cylinder 230 via a first circuit 229 whereas the secondactuator port 227 thereof is connected to the other chamber 233 of theoperating cylinder 230 via a second circuit 232.

The above mentioned first circuit 229 is provided therein with a lockingvalve 234, which is designed to assume a thrusting action in thedirection of communication under a pressure of the first circuit 229 anda thrusting action in the direction of blocking both by a spring 235 andunder a pressure of a pressure receiving portion 236. The pressurereceiving portion 236 has a pressure of the retention pressuregenerating chamber 231 of the operating chamber 230 exerted thereonthrough a circuit 238 provided with a throttle 237. Also, the pressurereceiving portion 236 is connected to a reservoir 242 through a drainpath 241 provided with a throttle 239 and a switching valve 240, and theswitching valve 240 is held at its blocking position by a spring 243 andis arranged to assume a thrusting action to its communicating positionunder a pressure of a pressure receiving portion 244.

Numeral 245 represents a main relief valve. A circuit 246 upstream ofthe main relief valve 245 is connected via a check valve 247 to the sideto the operating cylinder 230 of the locking valve 234 in each firstcircuit 229 and is also connected via the check valve 247 to each secondcircuit 232. This arrangement is so made that when the highest pressurein each first circuit 229 and each second circuit 232 exceeds a presetpressure of the main relief valve 245, the latter may operate so as tobe relieved.

With such an arrangement adopted, only a single main relief valve aloneis made sufficient for use in a circuit assembly provided with aplurality of operating cylinders.

The above mentioned directional control valve 223, when no pilotpressure is acting thereon, will act to block both communication betweenthe pump port 224 and the first actuator port 226 and a communicationbetween the pump port 224 and the second actuator port 227, and is thusheld at its neutral position A which serves to communicate the loadpressure detecting port 228 with the tank port 225. It will be switchedto a first pressure fluid supply position B with a pressure fluiddelivered into a first pressure receiving chamber 248, and to a secondpressure fluid supply position C with a pressure fluid delivered into asecond pressure receiving chamber 249.

If the directional control valve 223 is switched to assume the firstpressure fluid supply position B, the pump port 224 will communicatewith both the first actuator port 226 and the load pressure detectingport 228, and the second actuator port 227 will communicate with thetank port 225. On the other hand, if it is switched to assume the secondpressure fluid supply position C, the pump port 224 will communicatewith both the second actuator port 227 and the load pressure detectingport 228, and the first actuator port 226 will communicate with the tankport 225.

The above mentioned pressure compensation valves 222 are each providedtherein with a check valve 250 and a pressure reduction valve portion251. The check valve 250 is designed to assume a thrusting action in thedirection of communication under an inlet side pressure acting on apressure receiving portion a and a thrusting action in the blockingdirection under an outlet side pressure acting on a pressure receivingportion b. Its inlet 252 is connected to the discharge path 221 whereasits outlet 253 is configured to communicate with the pump port 224 ofthe directional control valve 223.

The above mentioned pressure reduction valve portion 251 is designed toassume a thrusting action towards the direction in which the inlet 254and the outlet 255 may communicate with each other under a pressureacting on pressure receiving portion c, to block a communication betweenthe inlet 254 and the outlet 255 both by a spring 256 and under apressure acting on a pressure receiving portion d, and to assume athrusting action towards the direction in which the check valve 250 maybe blocked. Here, the pressure receiving portion c is connected to theload pressure detecting port 228 of the directional control valve 223,the pressure receiving portion d is connected to the outlet 255, and theinlet 254 is connected to the discharge path 221.

Further, the outlet 255 of each pressure compensation valve 222 isarranged to communicate with, and is connected to, a load pressuredetecting path 257 so that when a plurality of directional controlvalves 223 is at the same time operated to simultaneously actuate a likeplurality of operating cylinders 230, a highest load pressure may causeeach pressure compensation valve 222 to be brought into its set state,thereby enabling the plural operating cylinders 230 with varying loadpressures to be supplied with the pressurized discharge fluid from asingle hydraulic pump simultaneously.

It should be noted that the above mentioned hydraulic pump 220 is of thevariable capacity type in which its capacity is increased and decreasedby changing the inclination angle of its swash plate 258. A cylinder 260for controlling the inclination angle of the swash plate 258 is hereadapted to be supplied with the pump discharge pressure through acontrol valve 261, which is arranged to be switchable both under thedischarge pressure of the pump 220 and under the load pressure of theload pressure detecting path 257.

The pressure compensation valve 222, the cylinder 260 for rotating theswash plate 258 with a controlled inclination angle and the controlvalve 261, which are mentioned above, are here provided to allow thedischarge pressure fluid from the single hydraulic pump 220 alone to besupplied simultaneously to a plurality of the operating cylinders 230.Thus, it is understood that such a combination may be unnecessary eitherif only a single operation cylinder 230 is to be actuated or if there isno need to actuate a plurality of such operating cylinders 230 at thesame time.

Further provided is a pilot valve 262 for furnishing a pilot pressurefluid into the first and second pressure receiving chambers 248 and 249of each directional control valve 223. The pilot pressure fluid in thesecond pressure receiving chamber 249 is supplied to the pressurereceiving portion 244 of the switching valve 240.

An explanation will now be given with respect the operation of the abovementioned sixth embodiment of the present invention.

When a pilot pressure is not applied from the pilot valve 262 to thepressure receiving portion 248 or 249 of a directional control valve 223and thus the directional control valve 223 is held at its neutralposition A, the pilot pressure is not applied to the pressure receivingportion of the switching valve 240, either, so that the latter may be ina blocking state. Then, since the pressure (i. e., the retentionpressure) within the retention pressure generating chamber 231 of aoperating cylinder 230 acts on the pressure receiving portion 236 of thelocking valve 234 so that the latter may be held at a blocking positionby the spring 235, the pressure fluid may not flow through thedirectional control valve 223, and thus may not leak into any reservoir,thereby effectively preventing any operating cylinder from suffering aspontaneous fall. At this stage, however, the retention pressure, whichacts on the main relief valve 245, is lower than a preset pressuretherein and hence the main relief valve 245 will remain inoperative.

It should be noted at this point that if the pressure within theretention pressure generating chamber 231 tends to be abnormallyelevated due to an inertia load or an external force in the abovementioned state, any elevated pressure will then act on the relief valve245 through the check valve 247 and if it exceeds the preset pressure inthe main relief valve 245, the latter will act to be relieved to permitthe excessive pressure fluid to flow into the reservoir, therebypreventing the retention pressure from rising abnormally.

Also, when the pilot valve 262 is operated to supply the pilot pressurefluid into the first pressure receiving chamber 248 of the directionalcontrol valve 223, the directional control valve 223 will be switched tothe first pressure supply position B. Then, the pressure fluid will besupplied to the first circuit 229 to cause the locking valve 234 toopen.

Also, when the pilot valve 262 is operated to supply the pilot pressurefluid into the second pressure receiving chamber 249 of the directionalcontrol valve 223, the directional control valve 223 will be switched tothe second pressure fluid supply position C. At the same time, thepressure fluid will be supplied to the pressure receiving portion 244 ofthe switching valve 240 to bring the latter to the position ofcommunication. As a result, since the drain path 241 of the lockingvalve 234 is brought into a communication with the reservoir 242, thelocking valve 234 will open to allow the pressure fluid to flow out ofthe retention pressure generating chamber 231 of the operating cylinder230 into the tank port 225 of the directional control valve 223.

An explanation will next be given with respect to a specific structureof the directional control valve 223 for use in the present embodimentof this invention.

As shown in FIG. 16, the directional control valve 223 has a valve block270 formed therein with a spool bore 271 to which a pump port 224, afirst and a second load pressure detecting port 228-1 and 228-2, a firstand a second actuator port 226 and 227 and a first and a second tankport 225-1 and 225-2 are opened. A spool 272 is fittingly inserted inthe spool bore 271 and is slidably displaceable therein to establish andblock communications among these ports mentioned above. It should benoted here again that the first and second load pressure detecting ports(228-1 and 228-2) remain in communication with each other, here via afluid bore 320.

The above mentioned spool 272 is formed with a first and a second smalldiameter portion 273 and 274 and an intermediate small diameter portion275 which serve to block each of the ports mentioned above when thespool 272 is held at its neutral position as shown in FIG. 16. And, ifthe spool 272 is slidably displaced rightwards in FIG. 16 to take itsfirst pressure fluid supply position, communication will be establishedeach between the pump port 224 and the second load pressure detectingport 228-2 and between the first load pressure detecting port 228-1 andthe first actuator port 226. Then, the pressure fluid caused to flowinto the pump port 224 will flow into the first actuator port 226. Also,since the second actuator port 227 is then brought into a communicationwith the second tank port 225-2, the return fluid will now flow out ofthe second actuator port 227 into the second tank port 225-2.

If the spool 272 is slidably displaced from the state shown in FIG. 16towards the left hand side to take its second pressure fluid supplyposition, communication will be established each between the pump port224 and the first load pressure detecting port 228-1 and between thesecond load pressure detecting port 228-2 and the second actuator port227. Then, the pressure fluid introduced into the pump port 224 willflow into the second actuator port 227. Also, since the first actuatorport 226 is then brought into a communication with the first tank port225-1, the return fluid will now flow out of the first actuator port 226into the first tank port 225-1.

It is seen that a first spring box 276 is attached to one end surface270a of the above mentioned valve block 270 in the longitudinaldirection of the spool 272. The first spring box 276 is provided thereinwith a first spring bearing 277 and a second spring bearing 278 in sucha manner that the first spring bearing 277 may be in contact with boththe one end surface 270a of the valve block 270 and a step portion 272aof the spool 272 and the second spring bearing 278 may be in contactwith a bottom wall 276a and a step portion 279 of the spool 272. Aspring 280 is interposed between the first and second spring bearings277 and 278 such that the spool 272 may be held at its neutral position.Also, the interior of the first spring box 276 constitutes a firstpressure receiving chamber 248.

It is also seen that a second spring box 281 is attached to the otherend surface 270b of the above mentioned valve block 270 and its interiorconstitutes a second pressure receiving chamber 249.

Beneath the above mentioned valve block 270 there lie a check valvesection 250 which is constituted by a check valve bore 270e and a spool280 inserted therein, and a pressure reduction valve section 251 whichis constituted by a pressure reduction valve bore 270f and a spool 283inserted therein. The two spools 282 and 283 are aligned so as to opposeeach other. Also, the check valve section 250 and the pressure reductionvalve section 251 together constitute a pressure compensation valve.

A mating surface 270c of the above mentioned valve block 270 is joinedand connected with a mating surface 290a of a block 290, which is formedtherein with a first fluid bore 291 that is opening to the first matingsurface 290a and a second fluid bore 292. The first fluid bore 291 isdesigned to communicate with the first actuator port 226 that is openingto the mating surface 270c of the valve block 270 whereas the secondfluid bore 292 is designed to communicate with the second actuator port227 that is opening to the mating surface 270c of the valve block 270,Further, the first fluid bore 291 is provided with a locking valve 234.

The locking valve 234 includes a poppet 294 which is fittingly insertedin a valve bore 293 that is open to a second mating surface 290b of theblock 290 and which is being pushed by a spring 235 in its closingdirection. The spring 235 is received in a spring chamber 296 (whichrepresents the pressure receiving portion 236 in FIG. 15) that isdefined by a cap 295 which is mounted on the second mating surface 290bof the block 290. Also, the spring chamber 296 is open to the firstfluid bore 291 through an axial bore 297 and a narrow bore 298 (whichcollectively represent the circuit 238 including the throttle 237 inFIG. 15) that are formed in the poppet 294. Here, the axial bore 297 isdesigned to communicate with a port 300 of a valve bore 293 through anarrow bore 299.

The above mentioned block 290 is formed therein with a bore 301 that isopen to both the first mating surface 290a and the second mating surface290b. The bore 301 is designed to communicate with the above mentionedport 300 through a fluid bore 302, and also to communicate with areservoir 242 through a recess 303 that is formed in the mating surface290a of the block 290. Also, the bore 301 has a valve 304 fittinglyinserted therein which constitutes the above mentioned switching valve240. The valve 304 has one end facing a blind hole 305 in the cap 295and is arranged to be energized unidirectionally by a spring 306 so thata conical portion 307 formed at its other end may be in contact with asheet 308 so as to block a communication between the above mentionedport 300 and the above mentioned recess 303. Also provided is a springchamber 309 (which represents the pressure receiving portion 244 in FIG.15) that is designed to communicate with the above mentioned secondpressure chamber 249 through a bore 310 which is formed in the cap 295,a fluid bore 311 which is formed in the block 290 and a fluid bore 312which is formed in the valve block 270, so that the pressure fluid inthe second pressure receiving chamber 249 may flow into the springchamber 309 through the fluid bores 312, 311 and 310 to push up thevalve 304 and that the conical portion 307 may thereby be departed fromthe sheet 308 to establish communication between the port 300 and therecess 303, thereby permitting the pressure fluid within the first fluidbore 291 to flow into the reservoir 242. These components herecollectively constitute the above mentioned switching valve 240.

The above mentioned valve block 270 is formed therein with a drain port313, which is designed to communicate with the first fluid bore 291through a first check valve 314 and also to communicate with the secondactuator port 227 through a check valve 315. The first and second checkvalves 314 and 315 comprise a valve 316 that is attached by a spring 317to a sheet 318 under a pressure. Here, the first check valve 314 allowsthe pressure fluid to flow from the first fluid bore 291 to the drainport 313 whereas the second check valve 315 allows the pressure fluid toflow from the second actuator port 227 to the drain port 313.

As shown in FIG. 17, drain ports 313 as mentioned above, respectively,for a plurality of blocks 270, are designed to be each open to adjacentblock joining surfaces 270d at both sides of each block 270 in thedirection of its width and are designed to be interconnected by joiningtogether the respective blocks 270 of such a plurality of directionalcontrol valves 223. Also, a main relief valve 245 is attached to thevalve block 270 that is located at an end of a series of such valveblocks 270.

As will be apparent from the foregoing description, it can be seen thataccording to the sixth embodiment of the present invention in which amain relief valve 245 for preventing an abnormally elevated pressurewithin the retention pressure generating chamber 231 of an operatingcylinder 230 can be placed separately, it will be sufficient to provideonly a locking valve 234 and a switching valve 240 correspondingly toeach of the operating cylinders 230. And yet, since the main reliefvalve 245 can commonly act for a plurality of operating cylinders 230,the area of the site on which a unit for preventing a spontaneous fallof any of the plural operating cylinders 230 is mounted may be reduced.Also, since it suffices to provide such a single main relief valve 245alone, it can be seen that the equipment may be made less costly.

While the present invention has hereinbefore been described with respectto certain illustrative embodiments thereof, it will readily beappreciated by a person skilled in the art to be obvious that manyalterations thereof, omissions therefrom and additions thereto can bemade without departing from the essence and the scope of the presentinvention. Accordingly, it should be understood that the presentinvention is not limited to the specific embodiments thereof set outabove, but includes all possible embodiments thereof that can be madewithin the scope with respect to the features specifically set forth inthe appended claims and encompasses all equivalents thereof.

What is claimed is:
 1. A pilot pressure operated directional controlvalve in which a housing is formed therein with a spool bore having aplurality of ports, said spool bore has a spool fittingly insertedtherein so as to be slidably displaceable therein, said spool is adaptedto be slidably displaced both with a spring and under a pilot pressureled into a pressure receiving chamber, a spring box is attached to anend surface of said housing in a longitudinal direction of said spool,and said spring box is adapted to be formed with a hollow portioncontaining said spring and forming a pressure receiving chamber, and apilot pressure fluid inlet for communicating with said hollow portionand opening in the longitudinal direction of said spool, in which:pilotpressure fluid inlet is formed with a piping joint attachment hole; saidpiping joint attachment hole is constituted of a large diameter hole anda small diameter hole which are eccentric to each other; said pipingattachment joint is constituted of a fitting portion comprising a largediameter portion and a small diameter portion which are eccentric toeach other and a pipe connecting portion; said fitting portion isadapted to be fittingly inserted into said piping joint attachment hole;and a pressure plate is bolted with said spring box to act to preventsaid fitting portion from coming out of a piping joint attachment hole.2. A pilot pressure operated directional control valve as set forth inclaim 1, in which an interstice between an inner peripheral portion ofsaid piping joint attachment hole and an outer peripheral portion ofsaid fitting portions is adapted to be sealed by a sealing material. 3.An operating cylinder control apparatus, which comprises:a directionalcontrol valve; a locking valve which is disposed in a circuit forinterconnecting an actuator port of said directional control valve and aretention pressure generating chamber of an operating cylinder, whichhas a pressure receiving portion and a spring, which is adapted to bethrust in a direction of communication by an outlet pressure of saiddirectional control valve and the retention pressure within saidoperating cylinder and which is adapted to be thrust in a direction ofblocking by said spring and said retention pressure within saidoperating cylinder, said retention pressure acting on said pressurereceiving portion; a switching valve which is disposed between saidpressure receiving portion of said locking valve and a reservoir, whichis energized by a spring into a blocking position and which is adaptedto take a communicating position by means of a switching means broughtinto a position for communicating said circuit with a reservoir; and amain relief valve which is connected via a check valve to a side of saidoperating cylinder of said locking valve in said circuit, wherein saidmain relief valve is adapted to be connected to sides of a plurality ofoperating cylinders.
 4. An operating cylinder control apparatus, as setforth in claim 3, in which:a valve block equipped with said lockingvalve and said switching valve is connected to a valve block of saiddirectional control valve; an inlet side of said locking valve isadapted to communicate with the actuator port of said directionalcontrol valve; and each of said valve blocks is formed therein with afluid hole for communicating the pressure receiving portion on whichsaid pilot pressure of said directional control valve is acting with thepressure receiving portion of a side to said spring of said switchingvalve.
 5. An operating cylinder control apparatus, which comprises:adirectional control valve which is provided with a first pressurereceiving chamber, a second pressure receiving chamber, a pump port, atank port and a first and second actuator port, said directional controlvalve which when at its neutral position is adapted to block said firstand second actuator ports, which when at its first pressure fluid supplyposition is adapted to communicate between said pump port and said firstactuator port and to communicate between said second actuator port andsaid tank port, and which when at its second pressure fluid supplyposition is adapted to communicate between said pump port and saidsecond actuator port and to communicate said first actuator port andsaid tank port; a first circuit for connecting said first actuator portto a retention pressure generating chamber of an operating cylinder; asecond circuit for connecting said second actuator port to a secondchamber of said operating cylinder; a locking valve which is disposed insaid first circuit, which is adapted to be thrust in a direction ofcommunication both under a pressure of said first actuator port andunder a pressure within said retention pressure generating chamber ofsaid operating cylinder, and which is adapted to be thrust in adirection of blocking both by a spring and under the pressure withinsaid retention pressure generating chamber acting on a pressurereceiving portion of said locking valve; a switching valve which isdisposed in a drain path connected to said pressure receiving portion ofsaid locking valve, which is held at a blocking position by a spring andwhich is brought into a communicating position under a pressure at apressure receiving portion of said switching valve; a main relief valvewhich is connected to a side of said retention pressure generatingchamber and said second chamber of said operating cylinder by said firstcircuit and said second circuit via respective check valves, and saidsecond pressure receiving chamber of said directional control valve isconnected to said pressure receiving portion of said switching valve. 6.A pilot pressure operated directional control valve comprising:a housinghaving a spool bore formed therein, said spool bore having a pluralityof ports; a spool fittingly inserted in said spool bore so as to beslidably displaceable therein; a spring box attached to an end surfaceof said housing along a longitudinal direction of said spool, saidspring box comprising a first hollow cylindrical body having an open endwhich is attached to said housing, and a second hollow cylindrical bodyintegrally connected in parallel to said first hollow cylindrical bodyand having an open end opening in the longitudinal direction of saidspool and located at an opposite side of said spring box relative tosaid open end of first hollow cylindrical body, wherein said firsthollow cylindrical body defines an interior pressure receiving chamber,said open end of said second hollow cylindrical body defines a pilotpressure fluid inlet and said open end also serves as a piping jointattachment hole having an inner peripheral surface in the shape of aregular polygon, and said pressure receiving chamber is in fluidcommunication with an interior space formed in said second hollowcylindrical body; and a spring disposed in said pressure receivingchamber of said first hollow cylindrical body.
 7. The pilot pressureoperated directional control valve as claimed in claim 6, furthercomprising:a piping joint connected to said pilot pressure fluid inlet,said piping joint including a pipe connecting portion and a fittingportion having the shape of a regular polygon, said fitting portionbeing adapted to be fittingly inserted into said piping joint attachmenthole; and a pressure plate for securing said fitting portion in saidpiping joint attachment hole.
 8. The pilot pressure operated directionalcontrol valve as claimed in claim 7, wherein said pressure plate isbolted to said spring box.
 9. The pilot pressure operated directionalcontrol valve as claimed in claim 7, further comprising a sealingmaterial provided between a bottom portion of said piping jointattachment hole and an end surface of said fitting portion.
 10. A pilotpressure operated directional control valve comprising:a housing havinga spool bore formed therein, said spool bore having a plurality ofports; a spool fittingly inserted in said spool bore so as to beslidably displaceable therein; a spring box attached to an end surfaceof said housing along a longitudinal direction of said spool, saidspring box comprising a first open end which is attached to saidhousing, and a second open end which opens in the longitudinal directionof said spool and is located at an opposite side of said spring boxrelative to said first open end, wherein said spring box defines aninterior pressure receiving chamber, and said second open end defines apilot pressure fluid inlet which also serves as a piping jointattachment hole having an inner peripheral surface in the shape of aregular polygon; and a spring disposed in said interior pressurereceiving chamber.
 11. The pilot pressure operated directional controlvalve as claimed in claim 10, further comprising:a piping jointconnected to said pilot pressure fluid inlet, said piping jointincluding a pipe connecting portion and a fitting portion having anouter surface in the shape of a regular polygon, said fitting portionbeing adapted to be fittingly inserted into said piping joint attachmenthole; and a pressure plate secured to said spring box to prevent saidfitting portion from coming out of said piping joint attachment hole.12. The pilot pressure operated directional control valve as claimed inclaim 11, wherein said pressure plate is bolted to said spring box. 13.The pilot pressure operated directional control valve as claimed inclaim 11, further comprising a sealing material provided between abottom portion of said piping joint attachment hole and an end surfaceof said fitting portion.